Mostly shaft series hydraulic type. All hydraulic suspension devices are divided into three groups, front (or rear), left, and right, which are connected in series with oil pipes to form 3 closed loops, each connected to the valve of the control box. The trailer cargo platform load triangle formed by the fulcrums A, B, and C of the three loops is called "three-point support", which makes the cargo platform stably supported on a flat surface. A certain loop can be adjusted by the control box to make the heavy-duty train safely pass through a larger transverse slope or longitudinal slope; all loops are adjusted to make the piston of the hydraulic suspension device rise and fall, and the cargo platform rises and falls within a certain height range to facilitate passing a certain height. Standing traffic lanes or air obstacles, and can be leveled with the loading and unloading platform of large cargo, which is convenient for loading and unloading operations.
In addition, when a heavy-duty vehicle train equipped with a multi-axle series hydraulic suspension device passes on an uneven road surface, because each wheel is at a different position on the ground and receives different reaction forces, the piston of the hydraulic suspension device is raised and lowered. The liquid flows between the series circuits, while keeping the load balance of each hydraulic suspension device unchanged. In this way, the tires of any axle will not be overloaded.